1. Field of the Invention
The present invention relates to a helical compression spring for a vehicle suspension, and more particularly to the helical compression spring for use in a strut type vehicle suspension.
2. Description of the Related Arts
In general, a vehicle suspension is provided with a helical compression spring, which is generally designed to provide a coil axis that coincides with the direction of reaction force of the spring. Various types of vehicle suspension system have been known heretofore. Among them, a strut type suspension with a shock absorber employed as a strut for positioning a wheel is popular. According to the strut type suspension, however, due to a displacement between its load input axis and strut axis, a bending moment is exerted on the strut, with side force acting on a guide and a piston of the strut. This side force prevents the piston from sliding smoothly in the guide to act as the shock absorber. In order to compensate the bending moment, it has been known to arrange a coil axis of a cylindrical helical compression spring to be offset from the strut axis.
Since the arrangement between the coil axis of the helical compression spring and the strut axis of the shock absorber is determined geometrically, it has been required that the helical compression spring should be designed to coincide its coil axis with the direction of reaction force of the spring. In order to meet such requirement, various studies have been made, including the article xe2x80x9cApproaches to Minimizing Side Force of Helical Coil Springs in Suspension Designxe2x80x9d, presented by engineers including inventors of the present application, and published by Japan Society for Spring Research on Aug. 28, 1999, and the article xe2x80x9cApproaches to Minimizing Side Force of Helical Coil Springs for Riding Comfortxe2x80x9d, SAE Technical Paper No. 960730, pages 15-22, by the same persons as the engineers as mentioned above, both of which are hereby incorporated by reference. In those articles, it was aimed to minimize the reactive side force of the helical compression spring.
In Japanese Utility Model Publication No. 48-39290, proposed is a method for mounting on a vehicle a conventional cylindrical coil spring having a straight coil axis in an unloaded state, to be curved in the direction substantially vertical to the center line of the spring. Also proposed is a method for forming a coil spring with the center line thereof curved in advance in an unloaded state, and mounting it on the vehicle in such a state that the center line is straightened, to produce a moment by the reactive side force of the spring. According to those methods, the coil spring is mounted on the vehicle to be elastically deformed in the direction substantially vertical to the center line of the spring, to cause a side load which acts on a damper strut to produce a moment acting thereon in opposition to a moment caused by reaction force from a road, thereby to reduce the side force which will act on a bush and a piston of the damper strut.
Also proposed in British Patent No.1198713 is a strut type vehicle suspension, wherein a coil spring when installed in a suspension assembly is stressed, asymmetrically of its axis so as to impart to a shock absorber a bending moment in opposition to that imparted by a wheel support. In FIG. 2 of the British Patent, there is disclosed a helical spring which is coiled along a substantially straight line, and the two end turns of which are bent over by an angle xcex1xc2x0 towards one another. The planes which pass through a support surface of the unloaded helical spring converge towards the inside of the vehicle, whereas the spring plates connected to a housing and to a guide rod of the shock absorber, respectively, are arranged to parallel to one another. As a result, it is stated that a greater bias is produced on the half of the helical spring facing the outside of the vehicle than on the half facing the inside of the vehicle, so that a bending moment is exerted by the biasing force, opposite to the bending moment exerted by the wheel load on the shock absorber. FIG. 3 of the British Patent shows a helical spring which is coiled about an arc axis of the unloaded spring, and two support surfaces which extend obliquely at an angle to one another. When the helical spring is fitted between the parallel plates, and the longer surface line of the unloaded helical spring faces the outside of the vehicle, the outer half of the helical spring is compressed to a greater extent than the half which faces the inside of the vehicle. Referring to FIG. 4 of the British Patent, two plates are arranged to extend obliquely at an angle to one another, one being connected to the outer member of the shock absorber and the other to the inner member, the point of intersection of the two planes lying on the outside of the vehicle. And, a straight-coiled, cylindrical helical spring is compressed between the two plates, so that the half of the helical spring facing the outside of the vehicle is compressed to a greater extent than the half facing the inside of the vehicle.
Furthermore, a wheel suspension having a helical compression spring, the center line of which has an approximately S-shaped course in an unloaded state, has been proposed in a Japanese patent No.2642163, which claims a convention priority to one of two German patent applications to which U.S. Pat. No. 4,903,985 claims the convention priority, having FIG. 6 which is different from its corresponding FIG. 6 in the Japanese patent which was amended in the prosecution before the Japanese Patent Office. According to the Japanese patent, the invention was aimed to enable reduce a side force applied to a piston rod of a shock absorber to a great extent, in view of the fact that because tires are becoming wider and wider, hence shifting the wheel-to-road contact point outward, larger and larger angles between the line of support action and the shock absorber axis arise, so that the helical compression spring can not be positioned as obliquely with respect to the shock absorber axis as would actually be desired. In the Japanese patent, a helical compression spring, the center line of which is curved in an unloaded state, has been disclosed as a prior art in FIG. 5, and about which it is stated that the radius of curvature of the spring center line is constant, and that the line of the spring action is merely shifted from the center line of the helical spring, so that it is difficult to reduce the side force sufficiently. In other words, it has been concluded in the Japanese patent that the helical compression spring with its center line curved in the unloaded state is not to be employed.
With respect to the vehicle suspension, further reduction in size is required, so that it is difficult to compensate a bending moment exerted on a strut due to load from a road, by means of mere improvement to the strut and a supporting mechanism, using a conventional cylindrical helical compression spring. Rather, it is necessary to positively increase the reactive side force of the helical compression spring for use in the strut type suspension. However, it is not easy to produce a desired reactive side force of the conventional helical compression spring.
According to the structure for forming the coil spring with the center line thereof curved in advance in the unloaded state, and mounting it on the vehicle in such a state that the center line is straightened, as disclosed in Japanese Utility model Publication No.48-39290, it is difficult to install the spring on the strut type suspension appropriately, and hard to expect a desired effect. In that Publication, no appropriate type of the coil spring for use in that structure has been disclosed. Although the structure of the coil spring for use in the suspension of GB Patent No.1198713 has been disclosed, it is difficult to mount the spring and hard to expect a desired effect, too. In this respect, it is stated in the Japanese Patent No.2642163 as described before that it is difficult to achieve the sufficient reduction of the side force acting on the guide portion of the strut and the piston. Furthermore, it is difficult to manufacture the compression spring having the approximately S-shaped center line in the unloaded state as disclosed in the Japanese Patent No.2642163, which will cause increase in cost.
From the foregoing, it can be concluded that only by curving the helical compression spring as described above, the reaction force axis, or the line of the spring action, is shifted from the coil axis, or the center line of the helical compression spring, in parallel with each other, to be apart from the center line of the upper end plane of the helical compression spring. As a result, eccentric force will be exerted on the upper seat to deteriorate a structure for supporting the upper seat.
Accordingly, it is an object of the present invention to provide a helical compression spring for applying a desired side force to a strut of a vehicle suspension, when mounted on a vehicle, with a simple structure.
In accomplish the above and other objects, a helical compression spring according to the present invention is mounted on a vehicle body to be compressed between an upper seat and a lower seat, and includes a coil axis that is substantially curved at a predetermined radius of curvature in an unloaded state of the spring. A pitch of a lower end coil of the spring is set to tilt the end plane of the lower end coil at a first predetermined angle to the lower seat in the direction for shortening the longitudinal length of the spring at the inside of the curvature, and/or a pitch of an upper end coil is set to tilt the end plane of the upper end coil at a second predetermined angle to the upper seat in the direction for shortening the longitudinal length of the spring at the outside of the curvature.
Therefore, the helical compression spring may be embodied as follows.
Firstly, the helical compression spring may be formed with a coil axis that is substantially curved at a predetermined radius of curvature in an unloaded state of the spring, and formed with a pitch of a lower end coil that is set to tilt the end plane of the lower end coil at a first predetermined angle to the lower seat in the direction for shortening the longitudinal length of the spring at the inside of the curvature.
Secondly, the helical compression spring may be formed with a coil axis that is substantially curved at a predetermined radius of curvature in an unloaded state of the spring, and formed with a pitch of an upper end coil that is set to tilt the end plane of the upper end coil at a second predetermined angle to the upper seat in the direction for shortening the longitudinal length of the spring at the outside of the curvature.
Thirdly, the helical compression spring may be formed with a coil axis that is substantially curved at a predetermined radius of curvature in an unloaded state of the spring, and formed with a pitch of a lower end coil that is set to tilt the end plane of the lower end coil at a first predetermined angle to the lower seat in the direction for shortening the longitudinal length of the spring at the inside of the curvature, and a pitch of an upper end coil that is set to tilt the end plane of the upper end coil at a second predetermined angle to the upper seat in the direction for shortening the longitudinal length of the spring at the outside of the curvature.
It may be so arranged that the pitch of the lower end coil is set to tilt the end plane of the lower end coil at the first predetermined angle to the lower seat in the direction for shortening the longitudinal length of the spring at the inside of the curvature, and that the center of the end plane of the upper end coil is offset to the center of the end plane of the lower end coil in the direction of the inside of the curvature.
Or, it may be so arranged that the pitch of the upper end coil is set to tilt the end plane of the upper end coil at the second predetermined angle to the upper seat in the direction for shortening the longitudinal length of the spring at the outside of the curvature, and that the center of the end plane of the lower end coil is offset to the center of the end plane of the upper end coil in the direction of the outside of the curvature.
The upper end coil of the spring may be formed in a pig tail configuration, and/or the lower end coil of the spring is formed in the pig tail configuration.
The coil axis of the spring may be substantially curved in accordance with at least two radius of curvature in the unloaded state of the spring. Or, the coil axis of the spring may comprise a plurality of rectilinear lines connected to be substantially curved at the predetermined radius of curvature in the unloaded state of the spring.